Washington, Oct 2 : The clue to the missing link in planet-formation theory might lie in nearby stellar systems, according to a new study by astronomers from the University of Rochester.

Scientists study planets, which are in the process of formation to piece together information on how our own planet came to be formed.

But, so far, they have been unable to find evidence for one of the key stages of planet development, a period early in the planet's formation when it is only as large as Pluto.

Alice Quillen, associate professor of astronomy at the University of Rochester, and her team are now pointing to three nearby stars they say may hold “embryonic planets” – a missing link in planet-formation theories.

In an attempt to reveal this hidden phase of a planet's life, Prof. Quillen employed new Hubble Space Telescope imagery to measure the thickness of the dust disks that surround forming stars, and to calculate the size of the planets growing within.

According to Prof. Quillen, the results have helped paint a picture of a planet's earliest years, and give information on how our own small planet began its life.

Though scientists have inferred the presence of nearly 250 planets in the last decade, Prof. Quillen's method focuses on a unique aspect: the proto-planetary disk's thickness.

According to her theory, forming stars are usually surrounded by a disk of gritty dust, which provides the raw material for planet building.

The cloud of dust thins as the system ages, but if enough dust has clumped together, the “embryonic planet,’ as Prof. Quillen calls it, will knock the dust and grit into ever-more eccentric orbits. Over time, this will cause an otherwise razor-thin disk to appear puffed up.

“We're able to determine for the first time how large the bodies must be in a disk to scatter the dust the way we've observed,” said Prof. Quillen, one of the world's leading experts on the interaction between planets and stellar dust disks.

Using new Hubble images, her team measured the “puffiness” of AU Microscopii, Beta Pictoris, and Fomalhaut – three nearby stars with young disks positioned edge-on toward Earth.

As all three stars displayed a thicker disk than conventional models anticipated, so Prof. Quillen stepped beyond those models.

She said dust disks had a lifespan determined by a balance of how quickly the solar wind blew the dust away, and how quickly the largest “grit clumps” replenished the dust through their collisions.

Based on this balance, the size and age of a disk reveal how large the clumps inside must be, she said.

But, conventional theory never took a disk's thickness into account because until the Hubble images, astronomers had no way to measure it. Thus, the largest “clump” the model could predict was about a kilometre wide—a far cry from the fully grown planets that emerge from such disks.

Armed with the new images and her own models of dust dynamics, Prof. Quillen has now estimated how much mass is required to gravitationally scatter the dust to the thicknesses she observed.

“Those calculations pushed us into Pluto-sized bodies,” said Prof. Quillen.

Prof. Quillen is now looking for more young star systems to investigate with her model, but the criteria for candidates is quite strict.

According to her, the systems have to be young enough to still have their protostellar disks, but old enough to be forming the embryonic planets.

The systems must also appear edge-on from Earth and be near enough that Hubble can accurately discern the thickness of their disks.

At the moment, the three stars Prof. Quillen has already observed appear to be the only candidates that meet all the benchmark. (With inputs from ANI)